The present invention relates generally to microwave ovens and, more particularly, to devices for field stirring and shifting to provide uniform heating in microwave ovens.
In conventional microwave ovens, reflection of microwave energy by the walls of the oven heating chamber leads to the establishment of standing wave patterns or energy modes in the oven. Such patterns comprise locations of high and low electric field intensities which roughly correspond to locations of hot and cold spots in the heating field within the oven. This nonuniformity in the oven heating field results in undesirable nonuniform heating treatment of materials in the oven.
In order to improve the uniformity of heating effects in the oven it has generally been the practice to employ fixed or moving elements which allow a number of energy modes to be excited within the heating chamber of the oven. By adding together the electric fields of the different modes, a substantially more uniform heating field is obtained. A further improvement is to provide a field or mode stirring device, usually a rotating element having fan-type blades. The blades of the field stirring device reflect incident microwave energy as they rotate and so provide periodically shifting electric field patterns in the oven. The shifting heating patterns which result integrate timewise to provide more uniform heating effects in the oven heating chamber. However, the foregoing described devices do not entirely eliminate the problem of non-uniform heating effects in microwave ovens.
Another approach to providing uniform heating effects has been to employ rotating grids which affect the polorization and distribution of microwave energy present in the heating chambers of microwave ovens, periodically shifting the heating field pattern in the chamber and/or stirring up the energy mode patterns in the oven. However, these devices as previously disclosed, i.e. U.S. Pat. No. 2,618,735 and U.S. Pat. No. 3,189,722, although effective, have been bulky and have been subject to arcing and insulation failure at high power levels.